1. Field of the Invention
The present invention relates generally to a valve timing control apparatus and in particular to a valve timing control apparatus having a timing change mechanism for changing the timing of at least one of the intake and exhaust valves of an engine in accordance with the running condition of the engine.
2. Description of the Related Art
In the conventional combustion engine, the intake valve and exhaust valve operate to open and close air-intake and exhaust passages connected to the individual combustion chambers. The timing of both types of valves is controlled by the rotational phase of the crankshaft, which is in turn determined by the reciprocal movement of the pistons. The amount of intake or exhaust gas in any one of the combustion chambers depends on the angle of a throttle valve provided separately in the engine's air-intake passage, or generally on the speed of the engine. As shown in FIG. 8, during a cycle where the crankshaft angle changes 720 degrees, a short period of time exists when both of the intake and exhaust valves of a cylinder open at the same time. This so-called valve overlap helps to scavenge burned gases out of the combustion chamber of the cylinder and to pull fresh air and fuel into the combustion chamber.
Various apparatuses are available to ensure a variable valve timing in order to control the intake and exhaust amounts in the combustion chamber with a greater degree of freedom. Japanese Unexamined Utility Model Publication No. 62-59707 discloses one example of such an apparatus. As illustrated in FIG. 9, a cam shaft 91 in this apparatus drives intake and exhaust valves (neither shown). A variable valve timing mechanism (VVT) 92 disposed at the distal end of the cam shaft 91 operates using the pressure of the lubricating oil of the engine. The VVT 92 includes a cylindrical driving wheel 93 and a piston member 94, provided between this wheel 93 and the camshaft 91. The driving wheel 93 rotates on the power received from a crankshaft (not shown). The piston member 94 is coupled to the cam shaft 91 by a spline tooth 95. The piston member 94 is also coupled to the driving wheel 93 by another spline tooth 96. One of the spline teeth 95 and 96 is a helical tooth. As the driving wheel 93 rotates, the torque is transmitted via the piston member 94 to the cam shaft 91. When hydraulic pressure is applied to the piston member 94 to move this member 94 in the axial direction, the cam shaft 91 rotates relative to the driving wheel 93. By controlling the supply of the hydraulic pressure to the piston member 94 in accordance with the running condition of the engine, therefore, the piston member 94 moves inside the driving wheel 93 in the axial direction. As a result, the rotational phase of the cam shaft 91 with respect to the driving wheel 93 changes, altering the valve timing and adjusting the level of the valve overlap.
In this apparatus, a control unit 97 controls the supply of the hydraulic pressure to the VVT 92. The control unit 97 computes a target hydraulic pressure to supply the VVT 92 in accordance with the running condition of the engine. To obtain this target value, the control unit 97 controls the angle of an electromagnetic valve 99 provided in each hydraulic pressure passage 98. The angle control adjusts the level of the hydraulic pressure applied to the VVT 92 to change the rotational phase of the cam shaft 91.
A pressure sensor 100 provided in the hydraulic pressure passage 98 in this apparatus detects the level of the hydraulic pressure supplied to the VVT 92. The control unit 97 performs feedback control of the angle of the electromagnetic valve 99 in such a manner that the actual value of the hydraulic pressure detected by the pressure sensor 100 matches the target value of the hydraulic pressure according to the running condition of the engine. Through this control, the level of the valve overlap is controlled based on the running condition of the engine. Further, the control unit 97 computes the difference between the actual value of the hydraulic pressure and the target value. When the difference exceeds a predetermined value, the control unit 97 determines that an abnormality has occurred in the VVT 92 and restricts the subsequent application of the hydraulic pressure to the VVT 92. The control unit 97 stores the result of the decision in a backup memory so that this data can be used anytime for the maintenance of the VVT 92.
Another type of control apparatus has a sensor for detecting the rotational angle of the cam shaft (cam angle). In this apparatus, the control unit performs feedback control of the electromagnetic valve in such a manner that the actual value of the cam angle detected by the sensor matches the target value which is computed in accordance with the running condition of the engine.
Since both conventional control apparatuses simply perform feedback control of the electromagnetic valve in such a manner that the actual value of the hydraulic pressure or cam angle matches its target value, they have the following shortcomings. Both apparatuses use the lubricating oil to provide the hydraulic pressure to drive the VVT. Generally, the pressure given to the lubricating oil in the engine is obtained by the oil pump that is driven in responsive to the engine. Therefore, the hydraulic pressure applied to the VVT varies in accordance with a change in the engine speed. More specifically, when the engine speed is low in the two conventional apparatuses, the hydraulic pressure supplied to the VVT may drop or become low, delaying the response or actuation of the VVT. At a low engine speed, therefore, a longer time is required to change the cam angle to the target value than is needed at high engine speeds. It is likely that at low engine speeds or with low hydraulic pressure, complete actuation of the VVT will not occur within the time specified or allotted for its operation. Should this happen, the actual and target hydraulic pressure values will fail to converge, leaving the cam angle unacceptably large despite the proper functioning of the VVT. The control unit may, in response, determine that an abnormality has occurred in the VVT, and may consequently restrict the VVT's actuation. Unfortunately, with a restricted or nonfunctioning VVT, accurate valve timing can not be assured. This could jeopardize or cause the deterioration of the engine's operating conditions.
To overcome the above problems, an increased time period could be used to determine whether abnormalities exist in the VVT. Such an increased time period would allow the actual hydraulic pressure to reach or converge to the target pressure despite the operation of the VVT under low hydraulic pressure conditions. Despite the advantage of being able to accurately diagnose the functionality of the VVT, increasing the convergence time presents the following problem.
Should a malfunction develop in a VVT mechanism, in which convergence of actual hydraulic pressure to a preset target value was allowed to occur over a lengthened period of time, the VVT malfunction would not be immediately detectable by the unit controlling the VVT mechanism. In fact the control unit would not be able to detect such a malfunction until after the period of time set for such a convergence. Therefore, increasing the time needed for the actual hydraulic pressure to converge to a target value is not an effective way to determine VVT functionality.